This invention relates to inspection circuits of flat type light valve devices used for direct visual type display devices or projection type display devices. More specifically, it relates to light valve devices, and for example, inspection circuits of active-matrix liquid crystal display devices which incorporate integrated circuits, such as a liquid crystal panel formed of driving circuits unitarily into semiconductor thin films.
An active-matrix type liquid crystal display device has an extremely simplified operation principle, where switching elements are provided on each pixel, and in selecting specified pixels the corresponding switching elements are activated, and in a non-selecting state, the switching elements are deactivated. The switching elements are formed on a glass substrate constituting a liquid crystal panel, and it is therefore important to realize a method for more satisfactorily producing thin-film switching elements. For such elements, thin-film type transistors are generally used.
The conventional active-matrix device shown in a schematic circuit diagram in FIG. 6, comprises; pixels each arranged in a matrix shape in vertical and horizontal directions and formed of thin-film transistors 1 and electrooptic elements 3 such as liquid crystal elements, control signal lines 5 provided on gate electrodes of the thin film transistors 1, image signal lines 4 connected to source electrodes, an image signal line driving circuit 8 connected to the image signal lines 4, and a control signal line driving circuit 6 connected to control signal lines 5. The control signal line driving circuit 6 is mainly formed of shift registers, where each unit-bit output is connected to the signal lines 5. The image signal line driving circuit 8 is formed of the shift registers and sample hold circuits provided at every bit basis, and writes the image signals into the sample hold circuits in accordance with sampling signals from output of the shift registers.
The conventional light valve device has more than several hundreds of adjacent pixels arranged respectively in each of the vertical and horizontal directions, the quantity of pixels thus reaches an extent of one million and generally at least an area of more than 1 cm2. It is considerably difficult to produce such elements with a high production yield without any defect, and in general the produced elements are inspected in a form of the driving substrate before completion as a light valve device. For the most normal method of inspection, the measurement to determine acceptance or failure is performed in accordance with the current produced by applying a voltage through a metallic probe (hereinafter referred to as a prober) in contact with the electrodes of elements, or for the output voltage/current etc.
In the method described above, to confirm operation of the elements formed of a large number of pixels of the light valve devices or the like more than several hundreds of probers are required to be in electrical contact with the electrodes of elements at an interval corresponding to a pitch between pixels, and it is therefore difficult to obtain a reliable result in using the present technique. On the other hand, while measurement may be performed while moving a smaller number of probers, this however requires a long time for the measurement process and is not suitable for practical use.
For another method of inspection, it is considered by applicants to provide inspection circuits inside the elements. FIG. 6 shows an equivalent circuit diagram of the elements used in such inspection method, where transistors 23 having gate electrodes connected to the signal lines 4 are provided on signal output sections ranging from each driving circuit to the pixels, and in the inspection transistors 23 one-side terminals 24 are grounded and the other-side terminals are connected to common terminals 25 thereafter connected to a power supply 27 through a load resistance 26, such that an output of the load is then detected by the inspection transistors 23 at every bit. Signals from the driving circuit are applied to the signal lines 4 to turn ON the inspection transistors 23 and to produce current flow into the load 26, and with such current flow detected, the signal transfer to the signal lines 5 is confirmed. By observing timing of the current flow in synchronism with clock of the shift register, a bit relating to the operation can be determined to thereby detect a line on which a malfunction arises.
However, in the inspection circuit of the light valve device, if only one of the detecting FET's having several hundreds of bits comes to a turn-ON state, signals are detected in an output of a buffer amplifier, thus the inspection circuit of the light valve device does not determine on which of the bits the defect is generated in the case of the driving method of simultaneously originating signals for a plurality of bits. The image signal driving circuit generally produces the outputs at the same time from the entire lines. The present invention, which provides a function to control detecting operation at every bit basis, securely performs the detecting operation only at specified bits to exactly find a cause of the defect. With the malfunction securely determined, the defective components or parts are removed in the form of driving substrate, at the same time the cause of malfunction is fed back and thus reduces generation of such malfunction. The present invention also uses an electrical method, which enables rapid measurement.